In the conventional textile industry, card sliver is generally drawn and/or spun into a yarn which is subsequently formed into a fabric by either knitting or weaving.
In the non-woven textile industry, a plurality of carded webs are generally laminated and bonded together to form a fabric directly.
Since the uniformity of the fabric product is ultimately determined primarily by the uniformity of the carded web/sliver, it is highly desirable that their weight per unit length be exceedingly uniform. For most textile applications, the weight variation of every consecutive yard of web/sliver should not exceed about plus or minus 3 to 5 grains from the mean weight if a first class end product is to be achieved.
As a frame of reference, 3 to 5 grains correspond to about the weight of three United States postage stamps; each measuring about 7/8.times.1 inch. To make weight measurements of this order of magnitude requires sensitive laboratory grade instruments which is usually difficult to accomplish accurately in a production environment because minor outside variables such as air currents from the air conditioning systems or persons breathing near the scales can affect the results.
Therefore, to achieve this desired degree of uniformity on a production basis with a continuously moving product using fibers which are highly variable in their processing characteristics and behavior and using massive machines whose members possess high inertia is a problem to which considerable attention need be given. Many attempts by the prior art have been made to achieve this important, but elusive objective.
With a carding machine in good mechanical condition, there are fundamentally two factors which govern the instantaneous web weight delivered by it: the relative speed of the card feed roll with respect to the doffer cylinder, and the instantaneous batt or lap density entering the machine at the feed roll.
To maintain a constant and uniform card output weight, prior systems have attempted to either (a) monitor some characteristic of the web/sliver delivered, taken to represent weight, and vary the speed of the card's feed roll as required to hold this characteristic constant (hereinafter called, "After-Card Regulators"), or (b) to monitor some characteristic of the batt or lap, taken to represent density, and adjust some weight control mechanism in order to hold that characteristic constant (hereinafter called, "Before-Card Regulators").
After-Card regulators, by their very nature, are "closed loop" control systems in that they monitor downstream of the correction point and feedback a signal to adjust the speed of the corrector. Examples of these type systems are shown typically in U.S. Pat. Nos. 3,925,850, 3,157,915; 3,644,964; 3,852,848; 3,862,437, and 3,827,106.
It is well known that "closed loop" control systems usually require either wide dead bands or some form of dampening in order to remain stable. Otherwise, they will begin "feeding" off their own corrective signals and go into oscillation. A general rule of thumb is that, for most systems, the amount of delay should equal about 4 to 6 times the "time effective distance" between sensor and corrector.
In a card, this is not sufficient because the machine has a large capacity to store and release fibers within the carding points of the wire wrapped around the various cylinders. Because of the long distance between sensor and corrector, and the high amount of dampening needed to accommodate the system's "springiness" due to fiber storage, the "minimum effective correction length" of the art cited supra is somewhere on the order of 35 to 50 yards of web or sliver.
This is quite long for most textile applications because just one yard of card sliver appears in many thousands of yards of yarn. Devices of this type are capable of controlling long term weight "drifts" but because they have a tendency to oscillate or "hunt", they also generate "short term" variation which is deleterious to the quality of the yarn. Because these systems tend to create rather than eliminate short term weight variations, they are unsuitable to meet the objects of instant invention.
Prior art "Before-Card" regulators fall into two classes: card mounted batt formers and lap forming picker eveners ("scutchers").
The Chute Feeds taught by U.S. Pat. Nos. 3,709,406 and 3,889,319 are also "closed loop" systems in that they sense downstream of the correcting mechanism and feed back the control signal. Both are set point controllers.
U.S. Pat. No. 3,709,406 measures the thickness of the batt and varies the air pressure atop the stock column to attempt to hold the thickness constant. Obviously, this system requires a very slow response rate, or high degree of dampening to prevent the over-compaction of an entire chute full of fibers. This would result in hundreds of yards of "off weight" sliver and many, many thousands of yards of "off weight" yarn because compacted textile fibers do not have the capacity for total springback or recovery following the reduction or removal of a compaction load.
The chute feed of U.S. Pat. No. 3,889,319 reduces this weakness by employing a pair of "stuffing rolls" immediately up stream of the thickness sensing rolls. The stuffing rolls vary the number of fibers fed into the cavity (defined by the two pairs of rolls) as required to maintain a constant gap in the nip of the thickness sensing rolls. Using conventional diameter rolls, the distance from the nip of the sensing rolls to the nip of the correcting rolls is on the order of about 3 inches. Applying the closed loop "dampening rule" of 4 to 6 times delay means the "minimum corrective length" must be at least 12 inches of batt to remain stable. With conventional carding engine drafts, each inch of batt translates into about 2 to 3 yards of web/sliver which means the minimum corrective length in web/sliver is on the order of 25 to 35 yards.
These set point systems are suitable for controlling long term weight drifts but cause short term errors due to "hunting", and therefore, do not meet the objects of instant invention.
The well known "picker evener" illustrated typically by U.S. Pat. Nos. 3,400,432; 3,680,192; 3,109,204 and 3,231,940 represent a second form of "Before-Card" regulators. In these systems, a plurality of sensing plates or pedals are disposed along a feed regulating roll having a fixed axis of rotation. The pedals are mechanically linked with weights and levers or springs to integrate the displacement of each, and the resulting average displacement is applied directly to a speed variator to adjust the speed of the feed regulating roll.
Being an "open loop" system, the "picker evener" does not require dampening for stability and can respond fairly quickly to correct for short term errors.
Numerous attempts have been made to adapt the "picker evener" to carding engine applications without success because the characteristics of the batts measured and corrected by pickers is different from the batts or laps commonly fed conventional carding engines.
A picker usually produces enough laps to feed 6 to 12 cards and normally operates with production rates between 400 to 800 pounds per hour. The batt passing through its thickness sensing elements oftentimes has a weight on the order of 50 to 100 ounces per square yard or more.
Conversely, a conventional card usually operates at between 30 to 150 pounds per hour and experience has shown that the best carding quality is obtained when the batt or lap entering the card weighs on the order of 12 to 30 ounces per square yard.
As discussed in greater detail, infra, it is not practical to get a reliable, sensible control signal suitable for operating an open-loop control system over a wide range when conventional "picker evener" systems are applied to conventional cards because the reduced batt bulk is incapable of overcoming the frictional forces, lost motions, and minute structural member deflections characteristic of state of the art sensing mechanisms.
U.S. Pat. No. 2,725,599 teaches the application of a "picker evener" to a carding engine. Assuming this device works in practice, its success is undoubtedly attributable to the multiple laminations of the lap shown in the patent. By using multiple plys of lap, this device, in effect, reconstructs a thicker product for sensing and control. However, it is well known in the art that cross laid laps inherently contain a large amount of inch to inch, or short term variation due to the ridges and discontinuities caused by the criss cross patterns.
For the above reasons and others which will be apparent hereinafter, none of the prior art systems are suitable to meet the high standards of weight control which are the objects of the present invention.